Kevin Delevan receives a Sun 660 Polaroid camera for his fifteenth birthday. He soon notices that there is something strange about the camera: the only photographs it produces are of a malicious, feral black dog that seems to move closer with each shot as though to attack the person who is taking the pictures. On a recommendation, Kevin seeks help from Reginald "Pop" Merrill, the wealthy and unscrupulous owner of a junk shop in the town of Castle Rock, Maine. While just as unsettled by the phenomenon as Kevin, Merrill sees an opportunity to further his own interests; namely, selling the camera to a paranormal enthusiast for a great deal of money. He manages to switch out the camera for another of the same model, which Kevin destroys. Much to his dismay, however, Merrill cannot rid himself of the Sun as prospective buyers either dismiss it outright as a fake or decline to purchase it due to the discomfort and unease they feel upon viewing the photographs. Furthermore, Merrill finds himself increasingly compelled to use the Sun – the dog slowly advancing as it transforms into something more savage and monstrous with every picture he takes.
In the meantime, Kevin is plagued by recurring nightmares about the dog. Realizing that Merrill tricked him and the Sun was not destroyed, he sets out to prevent Merrill from taking any more pictures for fear that the dog will "break through" into the real world. By this point, the camera's influence over Merrill has caused him to lose his grip on sanity. After waking up in the middle of the night to find himself holding the Sun and repeatedly pressing its trigger, he resolves to smash it in the morning. However, he hallucinates that one of the cuckoo clocks hanging on the wall of his store is really the camera and smashes it instead. Guided by the illusion that he is repairing a clock at his workbench, Merrill starts taking pictures again. At this moment, Kevin and his father arrive to confront Merrill, but they are too late to stop him. The dog begins to tear its way out of the final photograph, killing Merrill in the process. Inspired by a scene from one of his nightmares, Kevin has brought another Sun along with him, and just as the dog is about to release itself, he takes its picture, trapping it once more in the "Polaroid world".
In the epilogue, Kevin gets a computer for his following birthday. In order to test its word processor function, he types "The quick brown fox jumps over the lazy dog." Rather than a printout of this text, the page reads, "The dog is loose again. It is not sleeping. It is not lazy. It's coming for you, Kevin. It's very hungry. And it's VERY angry."